Buy the extended battery for the galaxy nexus. It's the same size as the S3's stock, physical and mAh.

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I ended up buying an extra stock battery from the Verizon store when I picked up my phone today. I got it for $29.99. I think I got the discount because I have a standing employer discount on my account, 20% off all plans and 25% off most accessories. Damn, I gotta shop at Verizon more often!

As far as the extended battery, could you be more specific and possibly supply a link. After a brief Google search I haven't found any extended batteries with NFC for the Galaxy S III, although I'm sure they will come to market soon enough. But I must say that for now I am happy with having two stock batteries. We'll see how it goes.

As a side note, everybody, use extreme care when removing that back cover. I suspect the clerk at the store cracked mine when he put the SIM card in it. I noticed it a few hours later, returned to the store and after a bit of a hard time initially from this same clerk, had it eventually swapped out (after I asked to speak to the manager!).

The Galaxy Nexus' extended battery is 2100mAh, same as the S3 stock battery. The sizes are the same as well, so you may be able to find a nexus extended battery cheaper than a S3 battery. The batteries are the same size from what I've read.